I am using Firefox 4 Beta 5 and it's noticeably faster than Firefox 3.6, Mozilla is working hard to further enhance the browser performance and a new nightly developer build is now available for download with faster "JägerMonkey JavaScript Engine". The new javascript rendering engine is the code-name for the new JIT being developed for Mozilla's JavaScript engine where Jaeger means "hunter" in German.

Most Current Firefox Nightly Built can be downloaded from here - http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/.